Laryngeal adductor and post-inspiratory diaphragmatic muscle activities are prominent in the neonate during eupeic expiration retarding expiratory airflow and resulting in an elevation of end-expiratory lung volume above functional residual capacity. The hypotheses to be tested are that (A) In the neonatal animal, expiratory activation of laryngeal adductor and diaphragm muscles (mechanisms maintaining lung inflation during expiration) predominates even during hyperpnea and deflationary (laryngeal abductor and abdominal) muscle expiratory activities are elicited only at very high levels of ventilation. In contrast, deflationary muscle activities are elicited during hyperpnea in the more mature animal and inflationary activities are absent. (B) In both age groups, the pattern of expiratory muscle activations is determined to a large extent by mechanoreceptor feedback from the lungs and larynx with chemoreceptor activities playing a larger role with maturation. (C) The mechanisms described in hypotheses A and B are present during wakefulness and non-REM sleep but are not present during REM sleep. A series of protocols are proposed which will define the mechanisms which determine the balance between expiratory muscle activities during eupnea and hyperpnea. Expiratory activities of laryngeal adductor, diaphragm, laryngeal abductor and abdominal muscles will be assessed during hyperoxic eucapnea, hypercapnia and hypoxia in spontaneously breathing unanesthetized and decerebrate piglets. In all studies, both neonatal and more mature piglets will be studied to define maturational changes in expiratory muscle activations and the respiratory strategies employed during hyperpnea and in REM versus non-REM sleep. The influence on expiratory muscle activities of alterations in pulmonary and laryngeal mechanoreceptor afferent activities will be assessed during hyperoxic eucapnia to determine the extent to which each type of afferent activity influences the pattern of muscle activation. The interaction between chemoreceptor and mechanoreceptor feedback will be assessed. Developmental aspects of respiratory control are poorly understood. These studies will enhance our understanding of the control of airflow and lung volume in the neonate. They have direct relevance to the alterations in breathing strategies which occur with lung diseases such as Bronchopulmonary Dysplasia and Respiratory Distress Syndrome in which alteration of the mechanical characteristics of the respiratory system occur.